Many pacemakers and ICDs include one or more leads mounted in the atria for directly sensing atrial events, particularly P-waves. Reliable P-wave detection is required so as to determine the atrial rate for detecting atrial fibrillation and for controlling atrial pacing functions, such as dynamic atrial overdrive (DAO) pacing. A long-standing problem with atrial sensing is that electrical signals generated in the ventricles often appear within the atrial signals and can be misinterpreted as P-waves. The signals from the ventricles are referred to as far-field signals. Indeed, because the ventricles are considerably more massive than the atria, depolarization of ventricles generates R-waves having magnitudes far greater than P-waves. Even when sensed with leads mounted within the atria, the far-field R-waves often appear with at least the same magnitude as the near-field P-waves, making it quite difficult to reliably detect only P-waves. Note that, strictly speaking, P-waves and R-waves are features of a surface electrocardiogram (EKG). For convenience, herein, the terms P-wave and R-wave are also used to refer to the corresponding internal electrical signal component.
The far-field sensing problem is illustrated in FIG. 1, which shows a atrial bipolar tip-ring signal 1 derived from atrial tip and ring electrodes, along with a corresponding surface EKG cardiac signal 2. As can be seen, within the atrial signal, P-wave 3 is well represented. However, R-wave 4 also appears within the atrial signal, with about the same magnitude as the P-wave. Accordingly, detection circuitry designed to detect events within the atrial signal may have trouble distinguishing between the P-waves and R-waves and, in particular, may count both events as P-waves for the purposes of atrial rate calculation, likely resulting in the calculated atrial rate being twice the actual atrial rate. For ICDs configured to deliver a high energy cardioversion shock to terminate atrial fibrillation, the erroneous calculation of the atrial rate may result in a painful cardioversion shock being delivered even though none is actually required.
FIG. 2 illustrates a typical atrial sense amplifier used for sensing atrial signals. An algebraic difference between input tip and ring atrial signals is generated by differential amplifier 5, yielding the bipolar tip-ring signal (signal 1 of FIG. 1.) The bipolar tip-ring signal is smoothed by filter 6 then rectified by rectifier 7 before being routed into comparator 8, which also receives a sense threshold voltage along line 9. The comparator outputs a sense pulse signal indicative of whether the rectified bipolar tip-ring signal exceeds the threshold voltage. The analog bipolar tip-ring signal is also output for separate processing by other components. When using the circuit of FIG. 2 to process atrial signals, such as those shown in FIG. 1, both the near-field P-wave and the far-field R-wave trigger a sense pulse due to their significant voltage swings, resulting in erroneous detection of the atrial rate, erroneous overdrive rate adjustments, and possible delivery of an unnecessary cardioversion shock.
Accordingly, it would be highly desirable to provide a technique for properly rejecting far-field events from atrial signals and in particular for providing an improved atrial sense amplifier capable of far-field rejection for use in a pacemaker or ICD. It is to these ends that the invention is primarily directed.